Low permeability, naturally fractured reservoirs such as coal seam gas (CSG, coalbed methane or CBM) and shale gas reservoirs generally require well stimulation to achieve economic production rates. Coupling hydraulic fracturing and micro-proppant or graded particle injections (GPI) can be a means to maximise hydrocarbon recovery from these tight, naturally fractured reservoirs, by maintaining or improving cleat or natural fracture conductivity. This paper presents a summary of the National Energy Resources Australia (NERA) project "Converting tight contingent CSG resources: Application of graded particle injection in CSG stimulation" - which assessed the application of micro-proppants, providing guidance on key considerations for GPI application to CSG reservoirs. Over the last decade, laboratory research and modelling have shown the benefits of the application of GPI to keep pre-existing natural fractures and induced fractures open during production of coal reservoirs with pressure dependent permeability (PDP). Laboratory studies, within this study, provide further insight on potential mechanisms and key factors, including proppant size and optimum concentration, which contribute to the success of a micro-proppant placement. Accompanying numerical modelling studies will be presented that describe the likely fluidized behaviour of micro-proppants (e.g., straining models, electrostatic effects, and ‘screen out’ prediction). This paper outlines the necessary reservoir characterization, treatment considerations, and key numerical modelling inputs necessary for the design, execution, and evaluation of GPI treatments, whether performed standalone or in conjunction with hydraulic fracturing treatments. It also provides insight on the practical application of GPI efficiently into fracturing operations, minimizing natural and hydraulic fracturing damage effects, thereby maximizing potential production enhancement for coals, shales and other tight, naturally fractured reservoirs exhibiting pressure-dependent permeability effects.

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Many coal seam gas (CSG) reservoirs (also known as coalbed methane) can have low permeability, require stimulation to produce economic rates and often exhibit pressure-dependent permeability (PDP) behaviour. Defining PDP behaviour in coal using reservoir simulation is a non-unique problem based on the uncertainty in coal properties and input parameters. Recent research demonstrated that an integrated analysis coupling of a diagnostic fracture injection test analysis, hydraulic fracture modelling and reservoir simulation can better characterise PDP behaviour in order to evaluate stimulation effectiveness in coals (Johnson et al. 2020). The present work aims to improve the recently developed model by including multilayer and permeability anisotropy effects. A reservoir model with multiple coal layers is established in a pressure-dependent reservoir simulator, based on the image log interpretations. Permeability anisotropy in the formation is realised by introducing heterogeneous distribution of permeability in different directions. Modelling results indicate effects of aspect ratio between multilayers on the pressure distribution and production history. A lower permeability anisotropy ratio yields better well productivity, and higher stimulation is required to increase the stimulated reservoir volume to maximise gas recovery. The improved model and workflow are applicable to other CSG fields for defining key variables where hydraulic fracturing performance has been unable to overcome limitations based on pressure dependency, often accompanied by low-permeability behaviour. This workflow has applications in Australia and many areas (e.g. China and India) exhibiting low-permeability and PDP behaviour and where only typically collected field data is available.

Presented on Wednesday 18 May: Session 18 Coal permeability is the key discriminator in well completion selection in coal wells. Low productivity is often attributed to compartmentalisation and pressure-dependent permeability (PDP) effects. Often, vertical well hydraulic fracturing is used to enhance productivity from lower-permeability coals, however, several authors have noted that coal fracture treatments can generate a large unpropped area of stimulated reservoir volume (SRV) that is generated from natural fracture activation and pressure-dependent leakoff. The result of this study confirms previous studies (using radial, cartesian, and enhanced SRV analytic models) that graded particle or micro-proppant injections in conjunction with hydraulic fracture treatments can be a means to enhance coal productivity in PDP-affected or low-permeability coals. In this work, data from the Bowen Basin will be used to investigate the implementation and benefits of micro-proppants in conjunction with horizontal well, multi-stage, hydraulic fracture treatments. The calibrated model will be based on a Bowen Basin case incorporating petrophysical, diagnostic fracture injection test (DFIT), hydraulic fracture, and can utilise production data to constrain modelling parameters. To better understand and provide guidance on co-application of horizontal, multi-stage hydraulic fracturing in conjunction with micro-proppant injections, a range of factors will be evaluated in this model including initial permeability, permeability anisotropy, fracture half-length, area and conductivity of the enhanced region between fractures, lateral length, and the number of fractures. This model will demonstrate the effectiveness, economic benefits, and optimal number of fracturing stages based on the reservoir parameters. To access the presentation click the link on the right. To read the full paper click here

Coal permeability is the key discriminator in well completion selection in coal wells. Low productivity is often attributed to compartmentalisation and pressure-dependent permeability (PDP) effects. Often, vertical well hydraulic fracturing is used to enhance productivity from lower-permeability coals, however, several authors have noted that coal fracture treatments can generate a large unpropped area of stimulated reservoir volume (SRV) that is generated from natural fracture activation and pressure-dependent leakoff. The result of this study confirms previous studies (using radial, cartesian, and enhanced SRV analytic models) that graded particle or micro-proppant injections in conjunction with hydraulic fracture treatments can be a means to enhance coal productivity in PDP-affected or low-permeability coals. In this work, data from the Bowen Basin will be used to investigate the implementation and benefits of micro-proppants in conjunction with horizontal well, multi-stage, hydraulic fracture treatments. The calibrated model will be based on a Bowen Basin case incorporating petrophysical, diagnostic fracture injection test (DFIT), hydraulic fracture, and can utilise production data to constrain modelling parameters. To better understand and provide guidance on co-application of horizontal, multi-stage hydraulic fracturing in conjunction with micro-proppant injections, a range of factors will be evaluated in this model including initial permeability, permeability anisotropy, fracture half-length, area and conductivity of the enhanced region between fractures, lateral length, and the number of fractures. This model will demonstrate the effectiveness, economic benefits, and optimal number of fracturing stages based on the reservoir parameters.